Coupled hydromechanical elastoplastic framework to assess stress state and stability of unsaturated soil

In the conventional local factor of safety (LFS) method, the Mohr's circle for stress (MCS) is assumed to shift leftward along the stress axis while maintaining a constant radius as matric suction decreases. It leads to considerable errors in slope stability assessment under low suction conditions. To improve the accuracy of the LFS method at small matric suction, this paper proposes a coupled hydromechanical elastoplastic framework for evaluating the stress state and stability of each point within unsaturated soil. Based on a validation model utilizing a soil column sample, the simulated and experimental shear strengths under different matric suction (MS) levels are compared. Results show that the proposed framework can reproduce the shear strength of unsaturated soil obtained from the experiments. Subsequently, the coupled framework is applied to simulate a two-dimensional (2D) homogeneous slope model. The simulations demonstrate that at high MS, the radius of the MCS does not change. However, at small MS, the radius of the MCS increases significantly as it shifts leftward along the stress axis. This indicates an amplification effect of MCS (ranging from 21.5% to 25.7%) in unsaturated soil under small matric suction. Based on the correlation between shear stress and MS, three intervals of the MCS variation are identified: the reduction zone (RZ), amplification zone (AZ), and translation zone (TZ). A comparative analysis with two conventional methods, the shear strength reduction technique (SSRT) and the limit equilibrium method (LEM), demonstrates that the LFS method holds potential advantages for slope stability assessment and the analysis of collapse evolution in soils.